In the webhook API documentation the subscope attribute is defined as follows.
An object that contains an array of column Ids if you want to limit the subscription to a subscope. Specified when a webhook is created and cannot be changed.
But then the example has the following. That is not an array of column ids. That is a string in an array that has a comma separated list of column IDs. I assume that you did not want to include the quotes?
"subscope": {
"columnIds": [
"7318427511613316, 7318427511613123"
]
}
https://smartsheet.redoc.ly/tag/webhooksObjects